Cementing oil and gas wells

ABSTRACT

A cement method and composition for cementing a well penetrating subterranean formations and aqueous based drilling fluid containing at least one cement retarder characterized by a major proportion of the drilling fluid from the well as it was drilled; water; a lesser proportion of dry cementitious material; a minor amount of a dispersant that does not effect satisfactory set cement within an acceptable time interval; and an accelerator selected from the class consisting of acetic acid; the first 4 carbon esters thereof; acetamide; monoethanolamine; and diethanolamine. The first 4 carbon esters are the methyl; ethyl; propyl; both normal and isopropyl; and butyl ester, normal, isobutyl and tertiary butyl.

FIELD OF THE INVENTION

The present invention pertains to cementing oil and gas wells withdrilling fluid converted to a cementitious slurry and including both adispersant and an accelerator for the cement. The cement slurrycontaining the dispersant and accelerator is pumped into the space to becemented sufficiently to displace substantially all the drilling fluidand bond the desired elements in place, all preventing fluid migration.

BACKGROUND OF THE INVENTION

The prior art is replete with cementing compositions.

Specifically, U.S. Pat. No. 3,168,139 to H. T. Kennedy, et al; U.S. Pat.No. 3,499,491 to R. E. Wyant, et al; U.S. Pat. No. 3,557,876 to A.Tregasser; U.S. Pat. No. 3,887,009 to G. L. Miller et al; and U.S. Pat.No. 4,176,722, William N. Wilson disclose well cement compositions whichhave been formed, at least in part, by well drilling fluids. Other artis discussed and abstracted in the Information Disclosure Statementaccompanying this application.

Many prior art efforts to convert drilling fluid to cement material haveposed problems in causing increased viscosity and flocculation and slowsetting of the cement. In the co-pending application Ser. No.07/131,878, now U.S. Pat. No. 4,883,125 and Ser. No. 07/342,421, bothentitled "Cementing Oil and Gas Wells Using Converted Drilling Fluid",Wilson et al, there was described in more nearly complete detail theproblems frequently encountered. Also in application Ser. No.07/393,546, now U.S. Pat. No. 4,953,620 entitled "NEW CEMENTACCELERATIONS"; inventors Robert B. Carpenter and Williams N. Wilson,assigned to the assignee of this application, there was also disclosedsome of the problems with other accelerators for the cement slurry.

Accordingly, it is an object of this invention to provide an improvedcomposition and method of cementing a well penetrating subterraneanformations in which a large portion of the drilling fluid has beenconverted to cementitious slurry for use to prevent having to dispose ofthe drilling fluid and all of the problems related thereto; as well asprovide a cure for effecting a satisfactory set cement in an acceptabletime interval.

SUMMARY OF THE INVENTION

It is an object of this invention to provide at least one of theforegoing features not provided heretofore.

Specifically, it is an object of this invention to provide a method andcomposition for cementing a well penetrating subterranean formation andemploying a portion of the drilling fluid that has been converted tocement; supplying substantially all of the features delineatedhereinbefore as desirable and not heretofore provided.

These and other objects will become apparent from the descriptive matterhereinafter.

In accordance with one embodiment of this invention, there is providedan improved method and composition for converting well drilling fluidsto cement slurries for cementing at least a portion of the elements in awellbore penetrating subterranean formations.

In accordance with one aspect of the present invention, an improvedslurry is provided by using a drilling fluid converted to the cementslurry by the addition of cementitious materials; a dispersant tominimize the tendency for flocculation or gelation has a disadvantage ofnot causing the cement to set rapidly; and an accelerator that causescement to set rapidly even when it has in it retarders that may havebeen present in the drilling fluid.

The cementitious slurry provides an improved slurry that effects morenearly uniform and complete displacement of the drilling fluid, tends tocreate good bonding between the cement and the elements in the wellborewhen it is set.

In accordance with still a further aspect of the present invention, acement slurry or composition using drilling fluid is provided whereinthe cementitious materials are added along with the dispersant and anaccelerator. The dispersant solves some of the problems present withincreased gelation and flocculation whereas the accelerator solves someof the problems with not obtaining set cement within an acceptable timeperiod.

The accelerator is selected from a group existing of a small molecule offatty acid, the first four carbon esters thereof, a small moleculeamide, monoethanolamine and diethanolamine.

In the aforementioned Ser. No. 07/131,878, there was illustratedschematic diagrams of wellbore and fluid circulation system as well asschematic diagram of cement mixing and circulation system for a batch orcontinuous mixing of the cementitious material into the drilling fluidto convert it to a cementitious slurry. This type of equipment andwellbore is so well known that it is not deemed necessary to havedrawings in this application. Such applications are also shown in theprior art as shown on the Information Disclosure Statement and patentscited therein without drawings.

DESCRIPTIONS OF PREFERRED EMBODIMENTS

The conversion of well drilling fluids into cement slurries for purposessuch as cementing casing in the wellbore into the annulus and to thesubterranean formation penetrated and other cementing operations isattractive for several reasons. Specifically, a major portion of thedrilling fluid is not subject to waste disposal problems and regulationsand the conversion of drilling fluid to cementitious slurry minimizesthe handling of the drilling fluid in the cement slurry, it minimizesthe cement slurry preparation time and expense and separation betweenthe drilling fluid and the converted cementitious slurry is not requiredto be maintained as cleanly as with the cement slurries per se.

The conversion of drilling "mud" to a cement slurry is not without someoperational problems and undesirable compositional changes when this isdone. For example, the addition of cementitious material; such as thePortland cements, lime, silica, alumina, lime and magnesia, silica,alumina and iron oxide, or calcium sulfate and the like; cansubstantially increase the viscosity of the fluid mixture and result insevere flocculation. Efforts to circulate such mixtures through awellbore can result in highly unsatisfactory circulation rates, pluggingof the wellbore and breakdown of the earth formation in the vicinity ofthe wellbore and a failure of the cement slurry to properly admix andbond. Certain dispersants have been developed for use in drilling fluidsduring drilling operations including lignite and lignosulfonates, whichare retarders.

One dispersant which has been commercially used in drilling fluids thatare to be converted is a low molecular weight styrene sulfonic acidmaleic anhydride copolymer and the water soluble salt thereof. These aresometimes known as "SSMA". U.S. Pat. No. 3,730,900 to Perricone, et aldescribes several drilling fluids which are treated with such adispersant for stabilizing the rheological and fluid loss properties,particularly under high temperature in the wellbore in the presence offluid contaminants. As noted on the Information Disclosure Statement,other patents also disclose dispersants for drilling fluids and fluidspacer compositions. In spite of the state of the art as evidenced bythese references and as known to applicants, there has remained theproblem of effectively converting a drilling fluid to suitable cementcomposition and displacing the drilling fluid in the borehole, includingan annular space between the casing and the borehole wall in a manner,which provides effective occupancy of the area to be cemented with acomposition that will form an effective bond as desired.

Moreover, although the addition of certain proportions of certaindispersants reduce the tendency for flocculation or gelation, increasedinterest in dispersants and retarders have made manifest the need forbetter dispersants and/or better accelerators to get the desiredproperties. Specifically, the dispersants that has been employedheretofore have also been retarders that retard the setting time of acement slurry.

In the foregoing Ser. No. 07/131,878, there was disclosed referencenumerals with the figures and it is deemed unnecessary to duplicate thatspecific information. It is sufficient to simply note that a casing isnormally extended into a portion of the formation from a wellhead and asecond casing extends into the wellbore to form an annulus which mayinclude washouts or void areas. The casing may be surface strings and asecond string of casing extend to the wellhead and be adapted to be incommunication with a pump at the surface for circulating drilling fluidto the interior of the casing and up through an annulus to a returnconduit. Drilling fluid is conducted through the return conduit to astorage tank or pit and recirculated to the pump through a pumping meansand a conduit during normal drilling operation. Conventional drillingfluid equipment includes shale shakers, sand separators and relatedequipment that are not usually shown anyway in the interest ofsimplicity.

One method for converting a drilling fluid to cementitious slurry is tosimply introduce dry blended cementitious material into the drillingfluid, alone or after water is admixed thereto until the desired densityis obtained. A minor proportion of the dispersant with or without theaccelerant can be employed in this process. Specifically, from zero upto 100 percent or more of water can be added to the volume of thedrilling fluid to be employed. The cementitious material and drycomponent is then added. For example, the water based drilling fluid mayhave a density that is normally in the range of 9-18 pounds per gallon(ppg) as it is being circulated in the drilling of the well. Otherproperties of drilling fluid, well known and not needing descriptionherein, may perform the functions of the drilling mud. After dilutionwith the water of say from zero to fifty percent water; for example,about ten percent of the drilling fluid volume, the dispersant may beadded. The concentration of the dispersant is in the range of 0.5-10pounds per original barrel of drilling fluid based on a 42 gallon barrel(hereinafter termed "ppb"), and preferably less than about 5 ppb. Byadding the dispersant at the time of the conversion of the drillingfluid to cement slurry surprising improvement in the mixing of thiscement material into the drilling fluid has been realized.

An excellent dispersant has been shown to be the sulfonated styrenecopolymer with maleic anhydride and comprises a low weight styrenesulfonic acid-maleic anhydride copolymer commercially available underthe tradename NARLEX D-72 from National Starch and Chemical Corporation,Bridgewater, New Jersey. The dispersant may be preblended with the drycementitious material and other additives, if desired, and stored, or itmay be added to the drilling fluid during the addition of the dilutingwater. Moreover, the dispersant may also comprise selected quantities ofsulfonated styrene amide sulfonated styrene, itaconic acid or acombination of sulfonated styrene copolymer with one or more compoundsselected from a group consisting of polyacrylates, potassium salts,phosphonates and other co-polymers such as partially hydrolyzedpolyacrylamides. Moreover, it is contemplated that monomers such asmaleic anhydride, maleimide and dimethylmaleate may be added incombination with the selected copolymer.

In addition, the accelerator may be added. Preferably, it is added in anamount from 1-10 ppb. preferably about 3-5 ppb. to get the desired setto the cement in the desired time interval. Specifically, instead ofhaving to wait for 30 to 40 hours for the cement slurry to set up, itcan be set within 24 hours or less using the accelerator. Probably thebest accelerator is the formic acid esters which are priorly describedand claimed in an application assigned to the assignee of thisapplication. Herein, the accelerator that is compatible is selected fromthe class consisting of acetic acid; methylacetate, ethylacetate, thepropyl acetates, both normal propyl and isopropyl; and the butylacetates; including the normal butyl, isobutyl and tertiary butylacetates. Specifically, these small molecule acetates hydrolyze in thepresence of water at relatively elevated pH in the range of 8-12, togive satisfactory acceleration of the set. It is theorized that when anacetate resin is used, the acetic acid is formed by hydrolysis and itthen reacts on a minor scale with surrounding constituents to accelerateset of the cement. Regardless of whether the theory is correct or not,it is a fact that the smaller molecules of the fatty acid estersthereof, and small molecule amides, including ammonia and other smallmolecule amides, can effect acceleration of the set of the cement; ascan monoethanolamine and diethanolamine.

Concurrently with or following the addition of the diluting water withor without the dispersant and/or the accelerator to the drilling fluid,Portland cement is added in the range of about 100 ppb. to 600 ppb.Hydration rate control, such as calcium sulfate, can be used in therange of 10 ppb. to 100 ppb. of drilling fluid if desired. Fluid losscontrol compositions such as inorganic salts, calcium aluminate,lignosulfonates with or without organic acids and polymers such HEC,CMHEC, AMPS and acrylic acids may be premixed with the other materialsor with a resulting cement slurry.

The above-described composition may be further modified by the additionof silicas, such as silica flour, in the amount of up to 100 percent byweight of the Portland cement fraction of the cement slurry to increasethe high temperature stability thereof. Also, the other cementingmediums such as magnesium salt, calcium aluminate, and the abovementioned calcium sulfate can be added to control slurry thickeningtime, change the development rate and total compressive strength, aswell as to obtain any other desired properties.

The Portland cement may be any of the usual types of cementing materialssuch as described in Ser. No. 07/393,546. A cement material of the typedescribed in U.S. Pat. No. 3,499,491is commercially available under thename "C-Mix" and can be employed if desired for use in conversiondrilling fluids to cement slurries but in this invention, however, theusual abnormally high gelation of the drilling fluid can be controlledby the dispersant and the set of the cement can be accelerated by use ofthe accelerator. In pursuing the present invention it was decided totest the cement's composition with the addition of the dispersant andaccelerator additive in a water based lignosulfonate drilling fluidhaving a density of approximately 12.3 ppg. A satisfactory cement wasobtained.

The cement slurry composition has been tested and found to haveproperties that indicate that it could be pumped at a satisfactorycirculation rate.

The mud to cement technology (MTC) has been developed by the assignee ofthis invention and the assignee of this invention seeks additionalaccelerators.

Work has been completed by chemists in the laboratory and some of theconventional type accelerators could not be employed in thisapplication. On the other hand, several compounds have been obtained andput through preliminary screening tests and look encouraging.

Work to date illustrates that an improved cement composition and processhas been provided for cementing oil and gas wells and subterraneanformation voids or spaces requiring the displacement of a drilling fluidand the emplacement of a cementing material having requisite bonding andstrength. Recirculation of the cement slurry may be carried out toassure complete displacement of the drilling fluid with a material whichsets to provide the requisite compressive strength when desired. Thepreliminary testing is encouraging and drilling fluid converted tocement using class A cement in proportions of approximately 250 poundsof cement per barrel of the original drilling fluid with a dispersantand accelerator as indicated in the indicated concentration range haveshown a viscosity reduction and anti-flocculation characteristics andsatisfactory set in an acceptable time interval.

The overall process of cementing a space in a wellbore according to thepresent invention may also include the displacement of a drilling fluidwith a preflushing medium which will further assure that thedisplacement or removal of the drilling fluid and the bonding of thecement to adjacent structures is enhanced. For example, it iscontemplated that the drilling fluid may be displaced from the wellboreby pumping a preflush medium comprising water and other suitabledispersants with or without anionic or nonionic wetting surfactants andwith or without viscosifying material such as HEC, CMHEC, PHPA,bentonite, attapulgite, sepiolite and sodium silicate with or withoutcertain waiting material such as barite, hematite, illmenite and/orsand. It is acceptable to form a rheologically compatible medium fordisplacing the drilling fluid and then be itself displaced byconventional Portland cement slurry, even when made by converting thedrilling fluid to a cementitious slurry.

There is provided an improved well composition and process for cementingusing the drilling fluid or a major portion thereof which has beenconverted to cement composition by addition of one or more cementitiousmaterials, dispersant and accelerator as indicated. The recirculating ofthe drilling fluid-to-cement conversion composition through the wellbore to completely displace or convert the drilling fluid is within thepurvey of this invention.

A preferred embodiment of the method involves producing cementcomposition of batch process until all of the drilling fluid requiredfor the cementing operation has been converted. Alternatively, thematerials added to the drilling fluid to convert it to cement may becontinuously added in a stream of drilling fluid as it is circulated inthe wellbore. The process of recirculating the drilling fluid convertedto cement in a volume range of from 10 percent to 1000 percent of thedisplacement volume of the wellbore space provides substantial assurancethat all the drilling fluid has been displaced from the wellbore andthat washouts, voids or other imperfections in the cement jacket orannulus are minimized.

A desirable cement composition in accordance with this invention is onethat permits complete circulation of the fluid out of the wellbore andreplacement with the cement composition itself. Since this circulationmay normally comprise two to as many as ten complete displacements ofthe system volume which includes the wellbore, the mixing tanks or pitsand all the interconnecting conduits, it is desirable that thecomposition not commence thickening or setting until circulation iscomplete. It has been found that this can be controlled with theaccelerators of this invention whereas other accelerators are toostrong, or act too rapidly.

Having thus described the invention, it will be understood that suchdescription has been given by way of illustration example and not by wayof limitation, reference for the latter purpose being had top theappended claims.

What is claimed is:
 1. A method for cementing a wellbore penetrating anearth formation to which a conduit extends, said wellbore having a spaceoccupied by a fluid composition to be converted to cement for cementingsaid space to form a seal between spaced apart points in said formation,said method comprising the steps of:providing means for addingcementitious material and a dispersant to said fluid, circulating saidfluid and adding said cementitious material and said dispersant to aquantity of the fluid in predetermined proportions to form a settablecement composition comprising a major portion of the drilling fluid inthe well as it was drilled; and water; a lesser proportion of drycementitious material; a minor amount of a dispersant that does noteffect a satisfactory set cement within an acceptable time interval; anda compatible accelerator selected from the class consisting of aceticacid; the first four carbon esters thereof; and acedamide and fillingsaid wellbore with said cement composition.
 2. The method as set forthin claim 1 including the steps of recirculating a volume of the cementcomposition through said space.
 3. The method set forth in claim 2wherein said dispersant is selected from the group consisting of styrenesulfonic acid maleic anhydride, sulfonated styrene imide, and asulfonated styrene copolymer in combination with a polyacrylate, apotassium salt, a phosphonate and a partially hydrolyzed polyacrylamide.4. The method set forth in claim 3 wherein said dispersant is added in aproportion between 0.5 ppb and 10.0 ppb of the original drilling fluidand cementitious material is added to the fluid at the rate of about 100ppb to about 600 ppb of original drilling fluid.
 5. The method set forthin claim 4, further including the step of:adding at least twocementitious materials to said fluid to control the hydration rate ofsaid cement composition and selected from a group consisting of Portlandcement, calcium sulfate and calcium aluminate.
 6. The method set forthin claim 5, also including the step of:displacing drilling fluid fromsaid space with a preflush composition comprising water and a sulfonatedstyrene copolymer to form a rheologically compatible material fordisplacing said drilling fluid prior to filling said wellbore space withsaid cement composition.
 7. A method for cementing a space in a wellborepenetrating an earth formation wherein said space in said wellbore isoccupied by a drilling fluid, said method comprising the steps of:addingcementitious material and a dispersant to a quantity of said drillingfluid to form a cement composition comprising a major portion of thedrilling fluid from the well as it was drilled; water; a lesserproportion of dry cementitious material; a minor amount of a dispersantthat does not effect a satisfactory set cement within an acceptable timeinterval; and a compatible accelerator selected from the classconsisting of acetic acid; the first four carbon esters thereof; andacetamide; and circulating said cement composition into said wellboreand continuing the circulation of said cement composition through saidwellbore until a total displacement of cement composition through saidwellbore is equal to at least 100 percent to 1000 percent of the volumeof said space in said wellbore.
 8. A method for cementing a wellborepenetrating an earth formation to which a conduit extends, said wellborehaving a space occupied by a fluid composition to be converted to cementfor cementing said space to form a seal between spaced apart points insaid formation, said method comprising the steps of:providing means foradding cementitious material and a dispersant to said fluid, circulatingsaid fluid and adding said cementitious material and said dispersant toa quantity of the fluid in predetermined proportions to form a settablecement composition consisting of a major portion of the drilling fluidfrom the well as it was drilled; water; a lesser proportion of drycementitious material; a minor amount of a dispersant that does noteffect a satisfactory set cement within an acceptable time interval; anda compatible accelerator consisting of monoethanolamine anddiethanolamine; and filling said wellbore with said cement composition.9. A method for cementing a space in a wellbore penetrating an earthformation wherein said space in said wellbore is occupied by a drillingfluid, said method comprising the steps of:adding cementitious materialand a dispersant to a quantity of said drilling fluid to form a cementcomposition consisting of a major portion of the drilling fluid from thewell as it was drilled; water; a lesser proportion of dry cementitiousmaterial; a minor amount of a dispersant that does not effect asatisfactory set cement within an acceptable time interval; and acompatible accelerator consisting of monoethanolamine and diethanolaminecirculating said cement composition into said wellbore and continuingthe circulation of said cement composition through said wellbore until atotal displacement of cement composition through said wellbore is equalto at least 100 percent to 1000 percent of the volume of said space insaid wellbore.